Problem: In chemistry class, the girl to boy ratio is $7$ to $10$. If there are a total of $68$ students, how many boys are there?
Explanation: A ratio of $7$ girls to $10$ boys means that a set of $17$ students will have $7$ girls and $10$ boys. A class of $68$ students has $4$ sets of $17$ students. Because we know that there are $10$ boys in each set of $17$ students, the class must have $4$ groups of $10$ boys each. There is a total of $40$ boys in chemistry class.